This application is a 371 of PCT/EP99/03227 filed May 11, 1999.
The invention relates to a process for preparing corrugated paperboard adhesive by controlling the viscosity of the carrier and of the finished adhesive to a fixed value (constant).
Corrugated paperboard adhesives have broad application in the production of corrugated paperboard. They are used when corrugated paper is glued to the liner. The glue used is preferably native starch. Native starch is the cheapest starting material for corrugated paperboard adhesive. In order to be able to use it, it is generally prepared by the STEIN-HALL process (U.S. Pat. No. 2,102,937). In this process a carrier is first prepared (gelatinized starch), also termed primary starch, which is then enriched with native starch as glue, also termed secondary starch, and other substances (for example borax, wet strength agent and modified starches).
The carrier (primary starch) is generally prepared in a vessel which is situtated on a balance and in which first a defined amount of water is introduced (batch process). A defined amount of native starch then follows as carrier starch, which is suspended in the water which has been introduced. This starch is gelatinzed by addition of alkali, generally in the form of sodium hydroxide solution (starch has the property of cold gelatinization at a defined alkalinityxe2x80x94approximately pH 13). In the STEIN-HALL process, sodium hydroxide solution is added until all of the carrier starch is gelatinized. In practice, it has been found that it is frequently better not to gelatinize all of the carrier starch, but only a part thereof. In this case one speaks of the xe2x80x9cMinocar processxe2x80x9d. The size of this part is determined by the viscosity of the gelatinized carrier. In order to know constantly the viscosity of the carrier during the process. the viscosity must be measured online. Therefore, in the inventive proposal, an online viscometer is built into the makeup vessel. The controlled member x forms the viscosity, the actuator y forms the amount of sodium hydroxide solution added and the characteristic viscosity for each gelatinized carrier is a constant, the preset value w (xcex7=xcex7xe2x80x2).
Although a process, the Coromat process, is known, in which the viscosity of the carrier is also controlled by the addition of sodium hydroxide solution, in this case the conditions are considerably more complicated The critical difference is that the total amount of water is added at the start of the batch process. It is thus no longer possible to prevent further increase in viscosity by decreasing the alkalinity when the preset viscosity of the carrier is achieved. In order, nevertheless, to be able to control the system to a preset value w, in this process the preset value curve (this is not a constant here) is controlled upwards and downwards when the tolerance of the desired final viscosity is exceeded or undershot by more than xc2x11%. This shift in final viscosity then forms a new preset value curve. A disadvantage of this known process is that corrections must be constantly carried out to the preset value viscosity, and the desired preset value is virtually never achieved. This control led deviation becomes the greater the more initial parameters are varied, for example by using varying amounts of washwater and of ink wastewater. These waters can have very different pHs and can thus considerably effect the carrier viscosity.
According to the invention the known process is to be further developed and made more effective and more economic. According to the invention the viscosity of the corrugated paper board adhesive is measured and controlled during the production process using an online viscometer, that is to say in such a manner that the preset value w of the viscosity of the carrier is a constant xcex7xe2x80x2 which is achieved by adding alkali in a defined reaction time txe2x80x2 (preset reaction value). In this case an alkali pump is the actuator y. The controlled member x is the respective viscosity xcex7 which is established by addition of alkali. When the preset viscosity xcex7xe2x80x2 is reached at the end of the reaction time txe2x80x2, a further increase in viscosity is prevented by adding a defined amount of water (secondary water). After addition of adhesive starch (secondary starch) and further additives (borax), the product is stirred using a dissolver agitator until a final viscosity xcex7xe2x80x2 has been established. By means of the inventive process, corrugated paperboard adhesives having various viscosities can be produced. Each adhesive is based on a characteristic carrier viscosity and final viscosity which are preset as fixed values (constant). In this manner, the carrier viscosity and final viscosity remain independent of external influences. The respective predetermined viscosity is deposited in a formula management system.
In particular, by means of the inventive process, preferably corrugated paperboard adhesives may be produced reproducibly which comprise any desired proportions of washwater and flexography water By controlling the viscosity of the carrier, their effects on the viscosity of the carrier are ruled out.